Method of making a composite heat-exchanger tube



F. PENSOTTI Sept. 16, 1969 METHOD OF MAKING A COMPOSITE HEAT-EXCHANGER TUBE Filed March 30. 1966 United States Patent 8, 56/ 6: Int. Cl. F28f 21/00; B21d 39/00; B23p /26 US. Cl. 165-180 6 Claims ABSTRACT OF THE DISCLOSURE A method of making a composite heat-exchanger tube wherein an exteriorly finned outer tube, forming by extrusion or casting unitarily with its fins, is provided with an inner corrugated surface whose projections and recesses are formed during the extrusion or casting step. An inner tube is pressed outwardly until the material of its outer surface fills the recesses between the projections of the outer tube to lock the tubes together. A pair of outer members are provided in axial contiguity, while a packing ring is disposed in an annular chamber defined between the abutting ends of the outer members.

The present invention relates to tubular members for heat exchange apparatus and more particularly to a method of and an apparatus for providing a tube with extended surface elements, for example, fin like extensions. The invention has particular application in economizers for boilers.

In a heat exchange apparatus, for example, in economizers, a liquid is passed along the interiors of tubes and a gas is passed around the outside of the tubes in indirect heat-exchange relationship with the liquid in the tubes. To increase the heat transfer efiiciency of the tubes, extended surface elements or fins are formed on the ouside of the tubes to provide an increased surface area for the gas to contact.

It is well known that ferrous metal surfaces over which gases flow are subject to corrosion because of the presence of sulfur dioxide, water and other agents in the gases which etch and destroy the ferrous materials. Corrosion is particularly evident with apparatus such as an economizer wherein the metallic components draw heat from the gases to the extent that the gas temperatures are lowered below their dew point and condensation occurs on the outside surfaces of the tubes.

It is also well known that cast iron is particularly suitable and resistant to corrosion, and for this reason, it is conventional practice to protect the tubes by covering them with collars or sleeves made of cast iron, the sleeves having extended surface portions so that they also serve to increase the heat transfer surface of the heat-exchange units. For assembly of a heat exchanger, the collars or sleeves, ribbed or finned, may be expanded by heating and inserted over a steel tube, or in some instances may be cast in place over the steel tube. It is also known to cover the tubes by driving the collars or sleeves over the steel tube, or by expanding the tubes within the collars or sleeves, to obtain maximum contact between the tubes and collars or sleeves.

One disadvantage resides in the difiiculty in obtaining an intimate contact between the collars or sleeves and the tube, with cost of the assembly generally increasing with the degree of contact sought. Obviously a metal- 3,467,180 Patented Sept. 16, 1969 collar or sleeve preferably is made of cast iron and is lurgical bond is not accomplished by present methods and apparatus, since with these methods, tube outside surfaces do not reach proper bonding temperatures during the assembly sequence. As a consequence, most units have a thin air film or space between a substantial portion of the outside surface area of the tube and inside surface of the cast sleeves or collars. Ideally, to obtain maximum flow of heat across the walls of the tubes, there should be no gap between the collars or sleeves and the tubes, since the air space reduces heat conductivity between the tubes and collars or sleeves and limits the heat transfer efiiciency of the assembly.

Adding to the difliculty is the fact that inside diameters of a blank molded tube may vary considerably along the tube length, and large (tolerances) allowances on thicknesses in standard tubes are used. If a tube is coupled or assembled onto a sleeve in the condition in which it comes from the rolling mill or foundry, much breakage and many points of no contact will result, requiring, to obtain good results, very expensive working of the surfaces to be coupled and/or expensive coupling operations.

A primary object of the present invention is to provide a novel method for obtaining a finned or extended surface tube assembly, particularly for applications, such as with economizers, feed heaters or regenerators, wherein intimate contact is obtained between a tube and collars or sleeves, and using components having greater tolerances than presently allowed.

A further object of the invention is to provide a more economical method for obtaining an extended surface tube assembly with high heat-transfer coefficient.

In accordance with the invention, a tube which has an outside surface manufactured with greater than presently allowed tolerances is assembled with at least one sleeve or collar, preferably of cast iron, having an inside surface dimensioned to contact the tube outside surface. The sleeve or collar is provided with annular extended heat transmitting portions. On at least one of the contacting surfaces, of the tube or sleeve, a plurality of longitudinally extending serrations are formed uniformly disposed circumferentially around the surface. After inserting the tube within the sleeve or collar, the tube is expanded within the collar with tube metal deforming, by virtue of the serrations, filling all voids between the contacting surfaces. In this respect, the serrations are sufiiciently deep to facilitate and allow the flowing of tube metal into all voids between the surfaces.

The invention and objects thereof will become apparent on consideration of the following description, with reference to the accompanying drawings, in which FIGURES 1 and 1A illustrate the invention and steps of the method of assembly in accordance with the invention;

FIGURES 2 and 2A represent an embodiment in accordance with the method and apparatus concepts of the invention;

FIGURES 3 and 3A represent a further embodiment or modification in accordance with the method and apparatus concepts of the invention; and

FIGURE 4 is a section view of an extended surface assembly in accordance with the invention with two sleeves or collars on the tube in abutting relationship.

Referring to FIGS. 1 and 1A, the numeral 12 designates a tubular outside element or collar or sleeve provided with extended surface portions or fins 14, which cast into the proper shape, for instance in accordance with the method described in the Anselmini et al. patent, No. 2,855,643, granted Oct. 14, 1958. Disposed inside of the collar or sleeve 12 is tube 16, the tube and collar or sleeve each being provided with cooperating grooves or serrations 18. The serrations preferably extend longitudinally along the tube outside surface and collar inside surface and are uniformly disposed around the respective surfaces. The depths and other dimensions of the grooves or serrations depend upon the materials utilized, but known methods can be used for proportioning the grooves or serrations to obtain the flow of metal in a manner to be described.

In assembly, the grooves or serrations can be formed in the elements during manufacture, for instance by casting or extrusion, or can be machined in the respective surfaces, and the elements then are assembled by sliding the inner element, the tube, within the outside element, the sleeve or collar. Following this, the inside element or tube is expanded within the collar or sleeve, and the serrations are deep enough to allow, during the plastic ex pansion, yielding of the material forming the serrations and ensuing intimate adhesion of the elements through the corresponding serrations. If the collar or sleeve is cast iron, the flowing material is primarily derived from the outside surface metal of the tube.

In the embodiment of FIGS. 2 and 2A, only the inside surface of the outer sleeve or collar 20 is grooved or serrated, and during expansion of the inner tube 22, the material making up the outer surface of the tube flows into the depressions 24 in the sleeve or collar.

In the embodiment of FIGS. 3 and 3A, the reverse arrangement achieves the objects of the invention, namely grooves or serrations 26 in the outside surface of the tube 28. Expansion of the tube 28 causes a suitable deformation or flowing of tube metal resulting in intimate contact between the cooperating surfaces.

In the three schemes, the important aspect of the invention is to provide at least one of the contacting surfaces with serrations or grooves in the form of scorings, knurls and the like with the grooves being dimensioned so as to absorb excess material or replace lack of material in the corresponding surface of the other member. This dimensioning should also take into consideration variations in the thickness of the tube wall, variations in tube inside diameter, and other tolerances generally used in machine assembly operations, with the thought that the present invention allows greater than average tolerances.

It should be noted that the arrangements of FIGS. 2-2A and 3-3A do not result in the intimate contact achieved with respect to the arrangement of FIGS. 1-1A, but nonetheless, constitute an improvement over prior art systems. Generally speaking all gaps can be minimized and substantially eliminated by suitably proportioning the depths of the serrations, to a greater extent actually than is shown in the drawings. The purpose of showing air spaces in the drawings is simply to demonstrate the method utilized.

In addition to obtaining intimate contact between components, the present invention has the advantage that breakage of the outside collar as a result of excessive material, or because of lack of material is avoided since the serrations and deformation of the serrations compensate for variations, tolerances and the like. As a further advantage, it is apparent that the invention avoids the need for close tolerance and critical manufacture necessary in the prior art to obtain maximum contact without interference of the components with one another in assembly.

A further feature of the invention is illustrated in FIG. 4. A suitable packing ring 30 is interposed between two successive extended surface elements (32 and 34), the ring being made of a material similar to the material of the tube 36. The packing ring is shaped to cooperate with recesses 38 and 40 formed in the adjacent cast members,

except that the inside diameter of the packing ring is larger than the outside diameter, after expansion, of the tube 36. This avoids breakage of the packing ring because of its reduced thickness during the expanding step of the tube. The final locking of the packing ring is obtained, before expanding the tubular member, by pressing together the two adjacent collar or sleeve elements.

Although the invention has been described with references to specific embodiments, variations within the scope of the claims will be apparent to those skilled in the art.

What is claimed is:

1. A method of making a composite heat-exchanger tube, comprising the steps of forming an exteriorly finned outer tubular member having an inner surface, and forming by extrusion an inner tubular member having an outer surface and receivable within the outer tubular member with said surfaces in close juxtaposition with one another, one of said surfaces being provided with a substantially uniform distribution over substantially its entire area of geometrically irregular projections and recesses during formation of the respective one of said members; telescopingly fitting said inner member into said outer member; and expanding the other of said members plastically to deform the material of the other of said surfaces into the recesses of said one of said surfaces and the projections of said one of said surfaces into said other of said surfaces to completely fill said recesses and mechanically interconnect said members.

2. The method defined in claim 1 wherein said outer tubular member is composed of cast iron and is provided with said projections and recesses along said inner surface during the casting of said outer member.

3. The method defined in claim 1 wherein said one of said members is extruded and is provided with said projections and recesses during the extrusion thereof.

4. The method defined in claim 1, further comprising the step of mounting a further exteriorly finned outer tubular member upon said inner tubular member in substantially end-to-end relationship with the first-mentioned outer tubular member, and bonding a packing ring to said inner tubular member upon expansion thereof while interposing said ring between portions of the adjacent ends of said outer member.

5. A composite heat-exchanger tube comprising an exteriorly finned cast-iron outer tubular member having an inner surface provided with a substantially uniform distribution over substantially its entire area of projections and recesses during the casting of said outer member, an inner tubular member having an expanded inner surface plastically deformed into the recesses of said inner surface whereby said projections penetrate the outer surface of said inner member, another exteriorly finned outer tubular member having an inner surface formed with said projections and recesses and disposed in end-to-end relationship with the first mentioned outer tubular member, said outer tubular members being formed with axially open annular clearances turned toward one another at their adjacent ends, and a packing ring mechanically retained on said inner tubular member and received in said clearances.

6. A composite heat-exchanger tube comprising an exteriorly finned extruded outer tubular member having an inner surface provided with a substantially uniform distribution over substantially its entire area of projections and recesses during the extrusion of said outer member, an inner tubular member having an expanded inner surface plastically deformed into the recesses of said inner surface whereby said projections penetrate the outer surface of said inner member, and another exteriorly finned outer tubular member having an inner surface formed with said projections and recesses and disposed in end-to-end relationship with the first-mentioned outer tubular member, said outer tubular members being formed with axially open annular clearances turned toward one another at their adjacent ends, said tube further comprising a packing ring mechanically retained on said inner tubular member and received in said clearances.

References Cited UNITED STATES PATENTS Chilton et a1 165133 X Farr et al 165-133 X Parris 165178 X Robertson.

6 3,100,930 8/1963 Nihlen et al 29 157.3 3,188,733 6/1965 Rickard 29-523 3,206,845 9/1965 Crump 29 523 FOREIGN PATENTS 5 850,058 9/1960 Great Britain.

PAUL M. COHEN, Primary Examiner US. Cl. X.R.

Black et 29507 10 29 157.3, 507, 523; 165133,183, 178 

